OBJECTIVES: The primary long-term goal of this study is to describe the way in which mechanical-physical features of the microvessel wall, neurogenic, and biogenic factors interact in regulating microcirculation, in conventional young and aged rats, gerbils, germfree and hypertensive rats. 1) Venular distensibility gradients required to evoke arteriolar and precapillary sphincters response will be analyzed in mesentery striated (cremaster) muscle, and brain (pial) vasculatures in rat and gerbil. The nature of the venous-arteriolar response will be explored, and the consequence of the microvascular diametric changes upon flow determined; 2) An assessment will be made of the influence of a selected number of naturally occurring biogenic molecules upon microvascular response to central neural stimulation in mesentery and skeletal muscle; 3) The extent, if any, of other central neural loci of transmitter(s) release e.g., nucleous coereleus, on cerebral (pial, cortical), and other tissue microcirculation will be examined.